Energy supply is one of the biggest challenges of the 21st century. Concomitant climate changes and limited future fossil-fuel supplies push towards the use of clean energy sources. Electrochemical energy storage is predicted to play a major role in the decarbonization of our energy. Batteries, which are electrochemical devices capable of storing energy through electrochemical reactions, are considered as one of the most promising technologies in response to future societal challenges such as electric transportation and stationary energy storage systems which can be used, for instance, as support for wind or solar sources.
With respect to negative electrodes, the use of carbonaceous electrodes is limited due to safety concerns and poor rate capability. To the opposite, titanium-based compounds are considered as strong candidates for safe negative electrodes in lithium batteries. Indeed, the operating voltage of this class of materials lies within the electrolyte stability region, i.e. >0.8V. This confers improved safety features to the battery, together with the desired absence of thermally unstable Solid-Electrolyte-Interface (SEI) layer as well as lithium plating on the anode.
Another interesting feature of titanium-based compounds is their ability to sustain high discharge/charge rates, which is required for high power applications such as electric vehicles. One approach commonly used to achieve enhanced rate capability is the reduction of particles size. A complementary approach comprises the modification of the structural arrangement through ionic substitutions.
Within the titanium dioxide family, the anatase (tetragonal, space group: I41/amd) form has been widely investigated due to its peculiar properties. Based upon the Ti4+/Ti3+ redox couple, a capacity of 335 mAh/g can be achieved. The anatase structure is built from TiO6 octahedra units linked through edge-sharing. This three-dimensional structure displays vacant sites suitable for lithium intercalation proceeding via a reversible first-order transition, i.e. from a tetragonal to an orthorhombic system. This phase transition behavior is characterized by a plateau region in the potential-capacity curve. Nevertheless, a solid solution property over the complete lithium composition range is preferred for practical applications. Indeed, this generally allows to avoid a nucleation process at high rate and an easier monitoring of the state of charge of the battery as compare to first-order transition materials.